1. Field of the Invention
The present invention relates to an optical pickup actuator and an optical recording/reproducing apparatus. In particular, the present invention relates to an optical pickup actuator for driving an objective lens mounted on the actuator in such a way that information can be recorded into, and reproduced from, an optical information storage medium, and an optical recording/reproducing apparatus having the same.
2. Description of the Related Art
Conventionally, a Digital Versatile Disk (DVD) is recorded/reproduced using an objective lens having a numerical aperture of 0.6 (in a recordable type, 0.65) and a light having a wavelength of 650 nm (or 635 nm). If such a DVD has a diameter of 120 mm and a track pitch of 0.75 μm, the DVD has a recording capacity of not less than 4.7 GB on one side thereof.
Therefore, such a DVD is not sufficient as a recording medium for recording moving image information of an HD (High Definition) grade, because a recording capacity of, for example, not less than 23 GB on one side of a DVD is required to record 135 minutes of moving image information in the HD grade.
In order to meet such a requirement for high-density record capacity, development and standardization for High Definition-DVD (HD-DVD), that is, next generation DVD are proceeding. The developments include using a light having a shorter wavelength than red color (that is, a blue light), employing an objective lens having a numerical aperture larger than 0.6, and having a narrower track.
In order to maintain a tolerance due to a tilt of an optical disc, the thickness of the optical disc may be reduced as the numerical aperture of an objective lens is increased for obtaining a high density. In consideration of an allowed tolerance due to a tilt of an optical disc, the thickness has been reduced from 1.2 mm for Compact Discs (CD's) to 0.6 mm for DVD's. For HD-DVD's, it is highly feasible for the thickness to be 0.1 mm. The numerical aperture of an objective lens has been increased from 0.45 (for CD's) to 0.6 (for DVD's). For HD-DVD's, it is highly feasible for the numerical aperture to be 0.85. In addition, it is highly feasible for a celadon light source to be employed as a light source for HD-DVD's in consideration of a record capacity. A problem encountered in developing optical discs of a new standard as described above is compatibility with existing optical discs.
However, a fairly sophisticated technique is required in developing and fabricating a one-piece objective lens having a high numerical aperture, for example 0.85. Furthermore, it is difficult to lengthen a working distance of such an objective lens to a distance suitable for an objective lens for a DVD, while allowing the objective lens to have such a high numerical aperture.
Therefore, it is required for a compatible type optical pickup capable of high-density recording/reproduction to separately include at least one objective lens for use in recording/reproducing a CD and/or a DVD and an objective lens for use in high-density recording, wherein the objective lens for use in high-density recording has a higher numerical aperture than the objective lens for use in recording a CD and/or a DVD, so as to address the problem of having proper working distance.
Meanwhile, since an actuator for such an optical pickup has a magnetic circuit configured in such a way that the actuator can be driven in both focus and track directions, the actuator serves to maintain the distance between an optical disc and an objective lens constant and to move the objective lens to a desired position (center of the track) in the track direction.
In order to address optical pickup compatibly when employing plural optical discs of different recording density, as described above, it is necessary to provide plural objective lenses each corresponding to one of plural optical discs of different recording capacity. Accordingly, an actuator applied to an optical pickup having plural objective lenses requires a magnetic circuit that can drive at least one of the plural objective lenses mounted on a movable part in focus, track and tilt directions while maintaining high sensitivity. Thus, complexity of construction is increased to meet the demands of HD-DVD.